As the fabrication process for semiconductors continues to increase the density of elements produced on the semiconductor, masks that are used in the process increase in complexity and the sizes of elements of the mask decrease. After production, and before being used, a mask needs to be inspected to ensure that it is within acceptable limits. The decrease in element size, typically to sizes below the diffraction limit of optical inspection apparatus, complicates the inspection process. The introduction of mask error enhancement factors, to compensate for the decrease below the diffraction limit, further complicates the inspection process. A number of methods for inspecting masks are known in the art.
Carl Zeiss Microelectronic Systems, of Jena, Germany, produce an “AIMS™ fab” aerial imaging system for optically inspecting masks. The system is stated to emulate the stepper/scanner under equivalent optical conditions. The system includes automated control for its numerical aperture, which is able to vary in the ranges from 0.3-0.76.
U.S. Pat. No. 6,757,645 to Chang et al., whose disclosure is incorporated herein by reference, describes a method and apparatus for inspecting a photolithography mask. The apparatus generates a first and a second simulated image of a defect area image. The method compares the first and second simulated images in order to determine the printability of an identified potential defect on the photolithography mask.